Large diameter gears are typically utilized in large comminution machines, e.g. crushing and milling machines, and in kilns. The applications in which such gears are utilized are typically high stress requiring the gear teeth and, obviously, the gear rim to have sufficient hardenability and strength to carry the transmitted load with minimum wear or reduced probability of fracture. Such large diameter gears are generally formed by a casting process, since, in a fabricated gear, the inner gear structure could not be easily welded to the hard gear rim. The reason for this is that typically the weldability of a material is inversely proportionate to hardenability. Therefore, if hardenability is required in the gear teeth as is generally the case for large diameter gears, it would be anticipated that, for fabricated gears, the quality of welds between the inner surface of the gear rim and the substructure of the gear would be poor. If the hardenability in the gear rim is compromised to allow ease of weldability, the gear teeth will generally not have sufficient hardness for the high stress applications.
Therefore, prior to the present invention a casting process was the only practical method to make large gears, i.e., gears having an outer diameter in excess of fourteen feet, for which the gear teeth would be subject to high Hertzian stresses. However, using a casting process to form such large gears is very expensive. In addition, there are only a few foundries in the United States, and in fact, worldwide, that are capable of making such large castings. Furthermore, such large castings would always be potentially be subject to casting defects. Finally, obtaining a large diameter cast gear is a very time consuming process for which it may take up to a year to cast a gear from the time the order was placed. For these and other reasons as described below it would be advantageous to have an alternative method of manufacturing such large gears.
It is an object of the present invention, therefore, to manufacture a gear rim, and further to attach the gear rim by welding to the associated gear substructure, for use in applications, such as material comminution, where exceptional gear teeth hardness is required, while not sacrificing the weld strength between the inner surface of the gear rim and the substructure of the gear. It is a further object of the present invention to manufacture such a gear rim by other than a casting process.
These and other objects are realized by the production of a bimetallic large gear rim in which there is a gear rim provided with two distinct layers, an inner layer having a high weldability and an outer layer having sufficient hardenability into which the gear teeth are cut.